The present invention relates to a method for cleaning a chemical storage tank. More specifically, the invention relates to cleaning and deodorizing a storage tank having mercaptan compound residue therein. Malodorous gas compounds are added to natural gas, which has no discernable odor, for providing a means of detecting a gas leak from a gas line. The class of malodorous compounds most widely used is a mercaptan-containing compound (mercaptan). Mercaptans are compounds similar to an alcohol where the oxygen molecule of the hydroxyl group has been replaced with a sulfur molecule and are recognized by their strong skunk-like odor. The addition of mercaptans to natural gas has historically taken place at an end users location such as at a town or large industrial complex. Recent changes in technology have facilitated adding mercaptan compounds to natural gas at the gas generating cite and not at the end users location. At the direction of the Environmental Protection Agency, the mercaptan holding tanks are being removed from the end users locations to prevent leaking.
The mercaptan storage tanks, having large quantities of reusable steel are frequently being recycled. However, upon crushing or shredding a storage tank at a recycling facility, mercaptan residue escapes into the atmosphere prompting complaints and false natural gas leak alarms by local residents. Thus, a need exists for removing mercaptan residue from these storage tanks prior to the recycling operation.
Several examples exist for removing malodorous sulfuric compounds from gases generated from sewage treatment plants, fertilizer plants or the like. One such example is U.S. Pat. No. 4,902,489 to Wantanabe which discloses washing a gas with an organic acid solution and subsequently treating with a lignin-sulfonate solution. A further example is U.S. Pat. No. 4,427,630 Aibe et al. which discloses deodorizing a sulfur containing gas with an absorbent consisting of activated carbon having bromine and an acid supported therein. A further example is U.S. Pat. No. 4,888,292 to Hata which discloses the use of autotrophic bacteria for deodorizing foul smelling substances from waste products. None of the prior art, however discloses a method for deodorizing a storage tank with a bacterial solution that can be continuously and repeatedly reused providing for economical deodorization of storage tanks.
The present invention is a method for deodorizing a storage tank having mercaptan residue disposed therein. A cleaning material is delivered to the storage tank. The cleaning material absorbs the mercaptan residue from the walls of the tank into the cleaning material. The cleaning material, having absorbed the mercaptan, is delivered to a recovery vessel. The mercaptan is sparged from the cleaning material with natural gas injected into the recovery vessel.
The advantage of the present invention is the ability to continuously and repeatedly use the cleaning material over a period of months due to the sparging step. The sparging step purges bacteria in the cleaning material of the absorbed mercaptan allowing the bacteria to again absorb mercaptan upon reintroduction to the mercaptan storage tank.